Spectrometers are optical instruments used to measure electromagnetic radiation properties, e.g., light intensity, over a specific portion of the electromagnetic spectrum range from the infrared band to the gamma ray band. Normally, a particular spectrometer operates over a limited portion of this total range because different techniques are used to measure the energy in different portions of the electromagnetic spectrum. Spectrometers determine the distribution of spectral energy portions within the electromagnetic spectrum by measuring the intensity of radiation absorbed, reflected, or emitted by a material as a function of different wavelengths, separating the radiation measurements into energy bands, and indicating the relative intensities of the energy in each band. Typically, spectrometers use detectors other than photographic film to determine the distribution of radiation in a particular portion of the electromagnetic spectrum. One example of a common detector used in a spectrometer is a charge coupled device (CCD).
Spectrophotometers are spectrometers that measure the ability of a material's surface to reflect spectral energy, i.e., the spectral reflectance of the material. While the measured spectral energy is usually in the visible band of the electromagnetic spectrum, the measured spectral energy may also be in the infrared, ultraviolet, or other bands. Spectrophotometers are commonly used to measure the spectral reflectance of the surfaces of printed matter, textiles, molded and machined parts, etc., for quality control and/or process characterization. A spectrophotometer typically measures integrated spectral reflectance in a small circular area, e.g., an area four to eight millimeters in diameter, thereby providing only one number to represent what may in reality be spectral energy at a plurality of different wavelengths, each of which varies in intensity at different locations within the circular area. Even though four to eight millimeters is a relatively small area, for many surfaces, e.g., textiles, spectral reflectance can vary widely over a surface area of this size. Hence, it is difficult to use a single spectrophotometer measurement to precisely determine how reflected spectral energy varies over a particular surface area.
One way to measure reflected spectral energy whose intensity in different bands varies over a surface is to take multiple, simultaneous samples across a grid of locations on the surface. Preferably, the spacing of the locations in the grid is small enough to provide information about the spatial variation of the surface's spectral reflectance. One way to take multiple, simultaneous, closely spaced samples over a surface is to use an image scanner. An image scanner scans a surface by sequentially scanning rows of locations and building up a grid of locations row by row. Typically, in an image scanner, light reflected from a row of locations strikes a row of sensors that measures the reflected light. A sensor row of an image scanner often comprises one or more charge coupled devices (CCDs). A charge coupled device (CCD) is an integrated circuit comprising a plurality of similar elements arranged in an array. Preferably, the elements in a CCD used in an image scanner are arranged in a row, i.e., a one dimensional array. Each element in a row of elements in a CCD comprises circuitry including a light-sensitive diode and a capacitor. The light-sensitive diode converts radiant energy into an electrical charge proportional to the intensity of the radiant energy. The capacitor is connected to the diode such that the capacitor can accumulate charge from the diode over an exposure time. Circuitry external to the CCD converts the charge in each CCD element to a voltage that is converted to a digital value that is stored in an electronic memory device. The sensor row in an image scanner may comprise one CCD or may comprise a plurality of CCDs arranged end to end to form one sensor row of CCD elements.
Used conventionally, rows of locations on a surface are scanned by sequentially moving a sensor row comprising one or more CCDs in relation to the surface. Each row of locations is scanned by the sensor row and values are produced for each location. Sequential rows of values are accumulated into an image with each CCD element providing one picture element (pixel) in the image. It is also possible to scan a surface with an image scanner by moving the surface in relation to the sensor row. An alternative to a row of CCDs is a row of Active Pixel Sensors (APSs). An APS is a complementary metal oxide semiconductor (CMOS) device in which the sensing circuitry is combined with the recording circuitry.